When a CNC machine won't power up, throws multiple unrelated alarms, or reboots mid-cycle, the power supply is the first thing to check — and the last thing to guess at. A failing PSU can produce symptoms that look like drive faults, I/O errors, or control crashes, leading to expensive component swaps that don't fix the problem.
Here's how to diagnose a CNC power supply fault systematically and replace only what's actually failed.
⚠️ Safety — You Are Working on Live Mains Voltage
Power supplies handle mains voltage (110–480V AC input). The DC output side is typically 5V, 12V, or 24V, but the input side is lethal.
- Lock out at the main isolator before opening any cabinet
- Verify with a multimeter that no voltage is present before touching terminals
- Capacitors in switch-mode power supplies can hold a dangerous charge for several minutes after power-down. Wait 5 minutes and measure DC bus voltage before working
- Never bridge or bypass fuses — a blown fuse is a symptom, not the problem. Find what caused it
- If you're not confident working on mains-voltage equipment, call a machine tool electrician
Signs Your CNC Power Supply Is Failing
Power supplies rarely fail without warning. Look for these symptoms:
| Symptom | What It Suggests |
|---|---|
| Machine randomly reboots or shuts down mid-cycle | Intermittent PSU output — likely failing capacitors or thermal shutdown |
| Multiple unrelated alarms appearing simultaneously | Supply voltage sagging under load — the control and drives are seeing brownout conditions |
| Control powers on but drives won't enable | The 24V DC supply for the drive enable circuit has failed or is overloaded |
| Screen flickering or dimming when spindle starts | The PSU can't handle inrush current — failing bulk capacitor |
| Burning smell or visible scorching near the PSU | Catastrophic failure imminent — power down immediately |
| Fan not spinning on the PSU | Overheating — the PSU will shut down on thermal protection. Replace fan or whole unit |
| LED indicators dim or off when they should be on | Output voltage has dropped below regulation |
Types of Power Supply in a CNC Machine
A typical CNC machine has at least three separate power supplies. Before ordering a replacement, identify which one has failed:
| Supply | Voltage | What It Powers | Failure Symptoms |
|---|---|---|---|
| Control PSU | 5V DC (±0.25V) | CNC control processor, memory, display logic | Control won't boot, screen dead, parameters lost |
| I/O PSU | 24V DC | PLC I/O, relays, sensors, contactors, drive enable circuits | Drives won't enable, tool changer stuck, alarms but control is running |
| Servo Drive DC Bus | 280–680V DC | Servo and spindle amplifiers | DC link undervoltage alarms, drives won't enable |
| Auxiliary PSU | 12V, 15V, or ±15V | Encoder circuits, analogue I/O | Position drift, encoder alarms, spindle won't orient |
Step-by-Step Diagnosis
Step 1: Check the Obvious First
Fuses. A blown fuse on the PSU input is common and easy to miss. Check all fuses in the power supply circuit — mains input fuses, DC output fuses, and any PSU-internal fuses. Replace with an identical rating. If it blows again immediately, there's a short circuit downstream.
Connectors. A loose or corroded connector between the PSU and the load can cause intermittent faults. Reseat all connectors with the power off, and look for green corrosion on pins.
Cooling. Is the PSU fan running? If not, the supply will overheat and shut down. Clean fan filters and heatsinks with compressed air — a PSU caked in coolant mist and swarf dust will thermally trip within minutes.
Step 2: Measure Output Voltage Under Load
Use a multimeter — not guesswork. With the machine powered on and the PSU under normal load:
- 5V rail must be 5.00V ±0.25V. Below 4.75V = failing PSU.
- 24V rail must be 22–26V. Below 22V = failing PSU or overloaded.
- DC bus varies by machine. Check the manual for the expected voltage range.
Measure at the PSU output terminals, not at the load. If the voltage is correct at the PSU but low at the control — you have a wiring problem, not a PSU problem.
Step 3: Isolate the PSU
Disconnect the PSU output from the load and power it up with no load. If the PSU output voltage is correct with no load but collapses under load, either:
- The PSU has failing capacitors and can't deliver rated current (replace PSU)
- Or the load has a partial short drawing excessive current (fix the load, not the PSU)
To distinguish: connect a known-good resistive load (e.g. a 24V lamp or power resistor) across the output. If voltage still collapses, the PSU is bad.
Step 4: Check AC Ripple
A failing PSU capacitor will show excessive AC ripple on the DC output. Set your multimeter to AC volts and measure across the DC output terminals. More than 100mV AC on a 5V rail, or 200mV on a 24V rail, indicates failing filter capacitors. The PSU needs replacement or repair.
💡 A PSU with excessive ripple can cause erratic behaviour that looks like a control fault. The CNC processor sees unstable voltage as corrupted logic levels — random alarms, memory errors, and unexplained crashes. If you're chasing an intermittent fault that makes no sense, check the PSU ripple.
Matching a Replacement Power Supply
When ordering a replacement PSU, you need to match:
- Output voltage — must be identical (5V, 12V, 24V, etc.)
- Output current — must be equal or greater than the original. A 10A supply can replace a 5A supply; a 5A supply cannot replace a 10A
- Input voltage — typically 100–240V AC single-phase or 380–480V three-phase
- Form factor — DIN rail mount, open-frame, enclosed, or chassis-mount
- Connector type — screw terminals, spade connectors, or proprietary plug
Never substitute a supply with lower current rating. It will run at maximum capacity, overheat, and fail — potentially taking downstream components with it.
PSUs We Stock
| Supply | Spec | Machine Compatibility |
|---|---|---|
| 24V DC PSU, 20A output | SP-500-24 replacement | Bridgeport / Hardinge VMCs — £245.45 → |
| Siemens PCU50 PSU | 6EW1811-8AA | Siemens 840D/840Di PCU50 IPC — £395.50 → |
| Cosel 5V 30A | LDA150W-5 | Fanuc, Mitsubishi, various — £152.85 → |
| Cosel 24V 4.5A | PBA100F-24 | General 24V CNC I/O supply — £219.85 → |
For the full range, browse our Power Supply category → — over 70 units in stock covering Cosel, Siemens, Deutronic, TDK-Lambda, and other industrial PSU brands.
⚡ After replacing a PSU, always check the output voltage with a multimeter before connecting the load. A replacement supply may ship with voltage adjust pots set to minimum or maximum. Adjust to the correct voltage before powering your control.
When the PSU Isn't the Problem
If you've replaced the PSU and the fault persists, the problem is likely downstream:
- Blown fuse was a symptom of a short on the 24V rail — check all solenoid valves and relay coils for shorts
- Control still won't boot — check the 5V supply wiring for voltage drop. Measure at the control PCB, not at the PSU
- Drives still won't enable — the 24V enable circuit may have a bad contactor or safety relay upstream